Inclusion Criteria:

          -  Have confirmed HR+, HER2+ in initial diagnostic tissue, early invasive breast cancer
             without evidence of disease recurrence or distant metastases

          -  Have undergone definitive surgery of the primary breast tumor(s)

          -  Have tumor tissue from breast (preferred) or lymph node

          -  Have received a minimum of four cycles of chemotherapy in either the neoadjuvant or
             adjuvant setting per standard of care therapy

          -  Have completed approximately nine months to one year of standard HER2-targeted therapy
             without evidence of disease recurrence (neoadjuvant/adjuvant combined duration)

          -  Have received one of the following eligible HER2-targeted adjuvant regimens AND be
             randomized within 12 weeks of completing the regimen:

               -  For participants treated with neoadjuvant therapy (chemotherapy administered with
                  HER2-targeted therapy): Single agent adjuvant ado-trastuzumab emtansine or

               -  For participants not treated with neoadjuvant therapy: Adjuvant pertuzumab with
                  trastuzumab

          -  Have high risk disease, defined by one of the following:

               -  For participants treated with neoadjuvant therapy (as defined above):
                  Pathologically detected axillary nodal disease in the surgical specimen

               -  For participants not treated with neoadjuvant therapy: Axillary node positive
                  disease meeting one of the following criteria:

                    -  Pathological tumor involvement in greater than or equal to (≥) four
                       ipsilateral axillary lymph nodes OR

                    -  Pathological tumor involvement in one to three ipsilateral axillary lymph
                       node(s) and at least 1 of the following criteria:

                         -  Histological Grade 3

                         -  Primary invasive tumor size ≥5 centimeters determined pathologically

        Exclusion Criteria:

          -  Have breast cancer with any of the following features:

               -  Disease recurrence or distant metastatic disease (including contralateral
                  axillary lymph nodes)

               -  Lymph node-negative status

               -  Pathological complete response from any prior systemic treatments for early
                  breast cancer

               -  Inflammatory breast cancer

          -  Have other medical conditions including:

               -  Previous breast cancer (Exceptions: Ipsilateral ductal carcinoma in situ [DCIS]
                  treated by locoregional therapy alone ≥5 years ago; contralateral DCIS treated by
                  locoregional therapy at any time)

               -  Other cancer being treated and/or not in complete remission within the last 5
                  years (Exceptions: Appropriately treated non-melanomatous skin cancer or
                  carcinoma in situ of cervix, bladder, or colon)

               -  Females who are pregnant or lactating

               -  History of venous thromboembolism

               -  Other serious medical conditions

          -  Have previously received treatment with:

               -  Any cyclin-dependent kinase (CDK)4 and CDK6 inhibitor

               -  Prior adjuvant treatment with immunotherapy, tucatinib, neratinib,
                  investigational HER2 directed therapy, or trastuzumab deruxtecan for treatment of
                  breast cancer

               -  Endocrine therapy (ET) (i.e., tamoxifen, raloxifene or aromatase inhibitor) for
                  breast cancer prevention (without diagnosis of breast cancer)

               -  Additional chemotherapy, anti-cancer ET, or HER2-targeted therapy beyond standard
                  of care therapy for their breast cancer at study enrollment